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CNIT 141: 5. More About Block Ciphers + Modular Arithmetic 2

A lecture for a college course -- CNIT 140: Cryptography for Computer Networks at City College San Francisco
Based on "Understanding Cryptography: A Textbook for Students and Practitioners" by Christof Paar, Jan Pelzl, and Bart Preneel, ISBN: 3642041000
Instructor: Sam Bowne
More info: https://samsclass.info/141/141_F17.shtml

CNIT 141: 5. More About Block Ciphers + Modular Arithmetic 2

1.
Understanding Cryptography – A Textbook for
Students and Practitioners
by Christof Paar and JanPelzl
www.crypto-textbook.com
Chapter 5 – More About Block Ciphers
ver. November 26, 2010
These slides were prepared by Amir Moradi, Christof Paar and Jan Pelzl
And modified by Sam Bowne
Last modified 10-2-17

2.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl2
Some legal stuff (sorry): Terms of Use
• The slides can used free of charge. All copyrights for the
slides remain with Christof Paar and Jan Pelzl.
• The title of the accompanying book “Understanding
Cryptography” by Springer and the author’s names must
remain on each slide.
• If the slides are modified, appropriate credits to the book
authors and the book title must remain within the slides.
• It is not permitted to reproduce parts or all of the slides in
printed form whatsoever without written consent by the
authors.

4.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl5
Block Ciphers
• A block cipher is much more than just an encryption
algorithm, it can be used ...
• to build different types of block-based encryption
schemes
• to realize stream ciphers
• to construct hash functions
• to make message authentication codes
• to build key establishment protocols
• to make a pseudo-random number generator
• ...
• The security of block ciphers also can be increased by
• key whitening
• multiple encryption

14.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl15
ECB Advantages
•No block synchronization between sender and
receiver is required
•OK if some blocks are lost in transit
•Bit errors caused by noisy channels only affect the
corresponding block but not succeeding blocks
•Block cipher operating can be parallelized
•Advantage for high-speed implementations

15.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl16
ECB Disadvantages
•ECB encrypts highly deterministically
•Identical plaintexts result in identical ciphertexts
•An attacker recognizes if the same message has been
sent twice
•Simply by looking at the ciphertext: traffic analysis
•Plaintext blocks are encrypted independently of previous
blocks
•An attacker may reorder ciphertext blocks which
results in valid plaintext

16.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Substitution Attack on ECB
• Once a particular plaintext to ciphertext block mapping xi → yi is known, a
sequence of ciphertext blocks can easily be manipulated
• Consider an electronic bank transfer
• the encryption key between the two banks does not change too frequently
• The attacker sends $1.00 transfers from his account at bank A to his
account at bank B repeatedly
• He can check for ciphertext blocks that repeat, and he stores blocks 1,3
and 4 of these transfers
• He now simply replaces block 4 of other transfers with the block 4 that he
stored before
• all transfers from some account of bank A to some account of bank B are
redirected to go into the attacker’s B account!17

17.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Example of encrypting bitmaps in ECB mode
• Identical plaintexts are mapped to identical ciphertexts
• Statistical properties in the plaintext are preserved in the ciphertext
18

20.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Cipher Block Chaining mode (CBC)
•There are two main ideas behind the CBC mode:
•The encryption of all blocks are “chained together”
•ciphertext yi depends not only on block xi but on all
previous plaintext blocks as well
•The encryption is randomized by using an
initialization vector (IV)
21

23.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl24
Substitution Attack on CBC
• Consider the last example (electronic bank transfer)
• If the IV is properly chosen for every wire transfer, the attack
will not work at all
• If the IV is kept the same for several transfers, the attacker
would recognize the transfers from his account at bank A to
back B
• If we choose a new IV every time we encrypt, the CBC mode
becomes a probabilistic encryption scheme, i.e., two
encryptions of the same plaintext look entirely different
• It is not needed to keep the IV secret! It can be sent in
plaintext.
• But it should be unpredictable

25.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Output Feedback mode (OFB)
• It is used to build a synchronous stream cipher from a block cipher
• The key stream is not generated bitwise but instead in a blockwise fashion
• The output of the cipher gives us key stream bits Si with which we can encrypt
plaintext bits using the XOR operation
• Image from Wikipedia (Link Ch 5a)
26

28.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Cipher Feedback mode (CFB)
• It uses a block cipher as a building block for an asynchronous
stream cipher
similar to the OFB mode
• The key stream Si is generated in a blockwise fashion and is also a
function of the ciphertext
• As a result of the use of an IV, the CFB encryption is also
nondeterministic
• It can be used in situations where short plaintext blocks are to be
encrypted
29

32.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Counter mode (CTR)
• It uses a block cipher as a stream cipher (like the OFB and CFB
modes)
• The key stream is computed in a blockwise fashion
• The input to the block cipher is a counter which assumes a different
value every time the block cipher computes a new key stream block
• Unlike CFB and OFB modes, the CTR mode can be parallelized since
the 2nd encryption can begin before the 1st one has finished
• Desirable for high-speed implementations, e.g., in network routers
33

36.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl38
Galois Counter Mode (GCM)
• Encrypts data in CTR mode, but also computes a Message
Authentication Code (MAC)
• By making use of GCM, two additional services are provided:
• Message Authentication
• the receiver can make sure that the message was really created
by the original sender
• Message Integrity
• the receiver can make sure that nobody tampered with the
ciphertext during transmission

39.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Exhaustive Key Search Revisited
• For DES, a 56-bit key encrypts a 64-bit block
• Only one key can decrypt a block
• In AES, a 128-bit or longer key encrypts a 128-bit block
• Only one key can decrypt a block
• If a cipher has a longer block size than key size, there's more than
one key that deciphers that block
• So several blocks must be tested to find the correct key
41

41.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl43
Increasing the Security of Block Ciphers
• In some situations we wish to increase the security of
block ciphers
• e.g., if a cipher such as DES is available in
hardware or software for legacy reasons in a given
application
• For AES, there are already three security levels
• 128, 192, or 256-bit keys
• No realistic attacks known for any of those levels
• No reason to increase the security with these
methods

42.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl44
Increasing the Security of Block Ciphers
• Two approaches are possible
• Multiple encryption
•theoretically much more secure, but
sometimes in practice increases the security
very little
• Key whitening
•Adding two additional keys

44.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Double Encryption
• A plaintext x is first encrypted with a key kL
• and the resulting ciphertext is encrypted again using a second key kR
• Assuming a key length of k bits, an exhaustive key search would
require 2k·2k = 22k encryptions or decryptions
46

45.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Meet-in-the-Middle Attack
• A Meet-in-the-Middle attack requires only 2k+2k = 2k+1 operations!
• It also requires 2k records of data storage for a look-up table
• Double encryption is not much more secure then single encryption!
47

46.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Meet-in-the-Middle Attack
• Phase I
• Brute-force the left half
• Save a table of middle values for each kL
• Phase II
• Brute-force the right half
• Find the kR value that matchs one of the middle values; that
determines kL
• Double encryption is not much more secure than single encryption!
number of encryptions and decryptions = 2k +2k = 2k+1
number of storage locations = 2k
48

48.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Triple Encryption
• Encrypt a block three times with three different keys
50

49.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Triple Encryption
• Meet-in-the-middle attack has one side with kL
• The other side has kR1 and kR2
51
Triple encryption effectively doubles the key length

51.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Key Whitening
• Makes block ciphers such as DES much more resistant against brute-force
attacks
• In addition to the regular cipher key k, two whitening keys k1 and k2 are used
to XOR-mask the plaintext and ciphertext
53

52.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Key Whitening
• DESX uses key whitening to make DES stronger
• In addition to the regular cipher key k, adds a whitening key k1
• k2 is calculated from key k and k1
• Even advanced attacks still take 288 calculations
• AES already includes key whitening
• Using a subkey before the first round and after the last round
54

53.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl
Quantum Computers
• Can crack a 128-bit key with only 264 calculations (Grover's algorithm)
• This is why AES has 192-bit and 256-bit modes
• They should still be unbreakable even when quantum computers become
available
• AES will remain secure
• Factoring a number becomes MUCH faster
• Exponential time changes to polynomial time (Schor's algorithm, link Ch
5c)
• Algorithms like RSA may become insecure, even for long keys
55

54.
Chapter 5 of Understanding Cryptography by Christof Paar and Jan Pelzl56
Lessons Learned
• There are many different ways to encrypt with a block cipher. Each mode of operation has
some advantages and disadvantages
• Several modes turn a block cipher into a stream cipher
• There are modes that perform encryption together together with authentication, i.e., a
cryptographic checksum protects against message manipulation
• The straightforward ECB mode has security weaknesses, independent of the underlying
block cipher
• The counter mode allows parallelization of encryption and is thus suited for high speed
implementations
• Double encryption with a given block cipher only marginally improves the resistance against
brute-force attacks
• Triple encryption with a given block cipher roughly doubles the key length
• Triple DES (3DES) has an effective key length of 112 bits
• Key whitening enlarges the DES key length without much computational overhead.